1. Field of the Invention
The present invention relates to a heating apparatus including a heat roller and a pressure roller which are mutually contacted at a surface and are freely rotatable, and an external heating roller which is contacted with the surface of at least one of the rollers and is rotated while heating such surface of the roller by an internal heat source, and adapted to heat a heated member while pinching and conveying the same between the aforementioned heating roller and the aforementioned pressure roller, and it also relates to a fixing apparatus for heat-fixing a toner image, formed on a recording material by an image forming apparatus of electrophotographic process such as a printer or a copying apparatus, to such recording material by utilizing the aforementioned heating apparatus.
2. Related Background Art
A known image forming apparatus of electrophotographic process is equipped with a fixing apparatus which fuse-fixes on a transfer material (heated member) serving as a recording material a toner image formed by toner constituted by a resin, a magnetic material, a coloring agent, etc. and electrostatically borne, to such transfer material, by applying heat and pressure while pinching and conveying the transfer material by a nip of a fixing roller (heating roller) and a pressure roller which are rotated in mutual contact.
A fixing roller employed in such fixing apparatus is formed by covering a surface of a cylinder for example of iron or aluminum with a heat-resistant elastic member for example of silicone rubber, and further forming on the surface a heat-resistant releasing resin layer constituted for example by polytetrafluoroethylene (PTFE) or perfluoroalkoxylalkane (FPA). The heat-resistant elastic member for example of silicone rubber is provided on the fixing roller in order to increase the thermal compliance to the toner, thereby providing a sufficient heat amount for fixing a toner layer having plural layers, under mixing thereof, onto the recording material.
The pressure roller is formed, so as to have a certain contact width in a pressed contact with the aforementioned fixing roller, by covering the surface of a cylinder for example of iron or aluminum with a heat-resistant elastic member for example of silicone rubber, and further forming on the surface a heat-resistant releasing resin layer formed for example by polytetrafluoroethylene (PTFE) or perfluoroalkoxylalkane (FPA).
Such fixing apparatus is usually provided with a heat source such as a halogen lamp inside the fixing roller or the pressure roller, and executes heating of the roller by turning on such halogen lamp (internal heating method).
In such internal heating method, in the course of a continuous fixing operation, in case the halogen lamp provided inside the fixing roller is turned on after the surface temperature of the fixing roller becomes lower than a predetermined control temperature, there is required a long time before the heat from the halogen lamp can reach the roller surface because of a low thermal conductivity of the heat-resistant elastic member provided on the surface of the fixing roller. As a result, the surface temperature of the fixing roller continues to lower by the fixing operation till the heat from the halogen lamp reaches the roller surface, thus eventually becoming lower than a minimum fixable temperature and resulting in a fixation failure. A similar drawback is also encountered in case of heating the pressure roller by the internal heating method, since the pressure roller is provided with the heat-resistant elastic layer.
Therefore, in order to avoid such fixation failure, there is employed a method of contacting a roller, incorporating a heat source and constituted by a material of a high thermal conductivity (hereinafter called an external heating roller) with the surface of the fixing roller, and rotating the external heating roller and the fixing roller under pressure contact thereby, directly heating the fixing roller from the surface thereof (external heating method).
In such external heating method, the surface of the external heating roller is controlled at a temperature higher than that of the surface of the fixing roller and the heat is directly transmitted from the external heating roller to the surface of the fixing roller at the contact portion (nip) therebetween, whereby the heat can be instantaneously supplied to the surface of the fixing roller in comparison with the aforementioned internal heating method and the fixing roller can be prevented from the temperature decrease. A similar effect can naturally be obtained also in case an external heating roller is provided on the pressure roller.
However, the aforementioned known external heating method is associated with a following drawback. The external heating roller has so-called straight shape in which the roller diameter remains same, along the longitudinal direction, namely in a central portion and an end portion, regardless of the shape of the fixing roller or the pressure roller, which is to be heated directly in contact with the external heating roller.
On the other hand, the fixing roller may have so-called inversely crowned shape in which the roller diameter is made larger, along the longitudinal direction, in an end portion than in a central portion, thereby applying a force to the recording material in the nip portion toward the ends portions of the roller and thus preventing a crease formation in the recording material at the fixing operation. In case the fixing roller of such inverse crown shape is contacted with the external heating roller of straight shape, since the external heating roller is given a constant pressure to the fixing roller, the pressure becomes larger in a portion of a larger diameter than in a portion of a smaller diameter, whereby, in the contact portion (nip) between the fixing roller and the external heating roller, the elastic layer of the fixing roller shows different crushed amounts. As a result, the nip assumes an inverse crown shape along the longitudinal direction, in which the nip width is larger in end portions than in a central portion. Thus, in the direct heat transfer from the external heating roller to the fixing roller at the contact portion (nip), the amount of heat transfer becomes larger in end portions where the contact width between the external heating roller and the fixing roller is larger, than in central portion, whereby the temperature on the surface of the fixing roller becomes uneven along the longitudinal direction.
On the other hand, in case the fixing roller has a normal crown shape and is contacted with the external heating roller of straight shape, the contact portion (nip) of the fixing roller and the external heating roller assumes a normal crown shape along the longitudinal direction, in which the nip width is larger in a central portion than in end portions, whereby, in the direct heat transfer from the external heating roller to the fixing roller at the contact portion (nip), the amount of heat transfer becomes larger in the central portion where the contact width between the external heating roller and the fixing roller is larger, than in end portion, and the temperature on the surface of the fixing roller again becomes uneven along the longitudinal direction.
Such unevenness in the surface temperature along the longitudinal direction of the fixing roller, if becoming larger, will lead to an unevenness of gloss of the fixed image along the longitudinal direction or an unstable behavior of the paper at the fixing operation, eventually resulting in creases on the paper or an image defect resulting from a flipping of the rear end of the paper sheet.
Also in case an external heating roller of straight shape is contacted with a pressure roller of normal or inverse crown shape, there will similarly result an unevenness in the temperature along the longitudinal direction on the surface of the pressure roller, thereby leading to an unevenness of gloss of the fixed image along the longitudinal direction or an unstable behavior of the paper at the fixing operation, and eventually resulting in creases on the paper or an image defect resulting from a flipping of the rear end of the paper sheet.